Influence of surface coatings and surface-active liquid lubrication on cyclic fatigue of nickel-titanium rotary endodontic files in an artificial canal

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The late 1980's witnessed the introduction of superelastic Nickel-Titanium (NiTi)alloy as an endodontic file. NiTi showed superior performance in root canal treatment compared to the conventional Stainless Steel files. After a few years of the use of rotary NiTi files, a greater tendency of fracture was noticed compared to Stainless Steel files, occurring unexpectedly and without previous permanent deformation. Recent research used electro-polished, smooth, triangular-shaped EndoSequence files in smooth glass surrogates of curved tooth-root canals, demonstrating extended times to file fracture during rotational bending at 300 rpm when the files were pre-coated with low-Critical Surface Tension (CST) silanes or lubricated in low-surface-tension surfactant solutions. A common feature was NiTi file fracture at a Surface Strain Amplitude of 3–4%, indicating that reduction of surface friction was by delay of surface crack opening. Earlier failure in NaOCl (bleach) lubricant was noted. That prior work tested files held at stepped, static locations in curved glass tubes. The aim of this follow-up research was to further evaluate the cyclic fatigue resistance of rough-surfaced, triple-U-shaped Protaper ® endodontic files, operated in more clinically relevant up/down filing motion, when their surface character also was modified through coating the files with two types of silanes, as "dry" lubricants. The first coating was Octadecylsilane (ODS) and the second was (3-Hepta-fluoroisopropoxy)propylsilane (3-HEPT). The cyclic fatigue of untreated control files and surface-modified files was tested dry and with five different irrigation solutions, some of which are commonly used in endodontic treatment. The irrigants used were distilled water, 5% sodium hypochlorite NaOCl, 17% ethylenediaminetetraacetic acid (EDTA), Surface Active Displacement Solution (SADS; an emulsion of water, butanol, and a surfactant), and Delmopinol (an aqueous solution of an aminoalcohol surfactant). Bent glass tubes simulated curved root canals, in which an AEU-20 Endodontic System ITR motor rotated files at 300 RPM and high torque. More than 300 samples were tested in this research. After testing, the files were viewed under the scanning electron microscope to evaluate the fractured area, and it was shown that all the tested files showed signs of abrasion on the edges of the fractured areas. Analysis of Variance (one way ANOVA) and Multivariate Tests were the main statistical methods to compare the number of cycles to fatigue fracture (NCF). NCF of the coated files was significantly higher than for files tested as received F (2,321)=6.887, P=0.001. When sodium hypochlorite was used as an irrigant, the files demonstrated the shortest fatigue life in the 45° bent glass tube due to high friction and corrosion of the nickel when the applied stresses were above the critical stress for martensite transformation. When low-surface-tension irrigants (SADS and Delmopinol) were used, the files demonstrated the longest fatigue life in the 45° bent glass tube due to better lubrication and friction reduction. It was confirmed that NiTi file failure occurs at mean surface strain values of 3–4%, irrespective of surface finish or degree of file curvature. Number of cycles to failure is significantly increased in the presence of surface-energy-reducing substances, both as coatings and as lubricants, separately or in combination. Failure is accelerated by corrosive attack of NaOCl (bleach) during rotational bending producing tensile-compressive transitions from the austenitic to martensitic atomic structures. Even in this latter case, reduction of crack-opening surface friction is the proposed beneficial mechanism.